Pumps having permanent magnet couplings are used in all cases where leakages are impermissible - i.e., in the displacement of poisonous, explosive and generally environmentally unacceptable media, and also where a high level of reliability and freedom from maintenance are to be achieved in comparison with conventional pumps having double action slide ring seals. The safe operation of such pumps therefore requires a reliable operation monitoring system free from maintenance.
It is known, for example, from the Leaflet "Chemienormpumpen mit Magnetantrieb" (Magnet Driven Standard Chemical Pumps), published by the Dickow KG, in 1991, that during operation these pumps generate in the metal antimagnetic gap-forming cup eddy currents which heat the product in the gap between the rotor and the cup. This heat is dissipated by an internal circulation or flow, so that it is impossible for the pumped medium to evaporate in this zone under normal operational conditions. If a malfunction occurs in which the pumped medium is either not supplied (dry running) or the internal circulation is interrupted or backed up (operation against a closed pressure side throttling member), at critical places the gap-forming cup becomes heated to temperatures up to several hundred degrees centigrade so that the bearings may be destroyed, the medium may become polymerized or there is a danger of explosion.
Due to the strong electromagnetic field and for structural reasons, hitherto the temperature of the gap-forming cup beneath the rotating magnets has never been monitored in such pumps. The standard temperature sensors, such as resistances or thermocouples, were disposed so far away from the heat source i.e., from the centers of the magnet elements, that as a result of the poor thermal conductivity of the antimagnetic materials of the gap-forming cup the temperatures actually occurring were not determined or monitored. In spite of the earlier temperature monitoring systems, therefore, the risks consequent on malfunction were always present.
Hitherto a starting safety device for antifriction bearing monitoring had been incorporated in such pumps in addition to the temperature sensors for the gap-forming cup. This safety device protected the cup against destruction if the outer magnets should be deflected and run eccentrically.